surface optical phonons. [14] As a dielectric interface, SiO 2 limits carrier mobility, and increases current-voltage hysteresis. [13,15] With a wide band gap (5.97 eV) and layered hexagonal Bernal structure (1.7% lattice mismatch to graphene) with dangling bond-free chemically inert surface, hexagonal boron nitride (hBN) is a near-ideal dielectric interface to graphene. The goal of this study is to investigate the graphene ISFET characteristics changed by introducing hBN under graphene. Incorporation of hBN as a dielectric in metal-gated graphene transistors has been shown to enhance performance by an order magnitude in terms of device mobility, reduced carrier inhomogeneity, lower extrinsic doping concentration, reduced chemical reactivity, and improved high-bias performance compared to devices fabricated with SiO 2 under graphene. [13,16] This has been attributed to increased flatness of graphene and opening of a band gap. Graphene has been shown to conform more to hBN than thermally grown amorphous SiO 2.[16e,17] This increased flatness aids charge homogeneity compared to the charge puddles on the order of 10 nm with 10 11 cm −2 charge variation that have been reported with SiO 2 device. [13,18] Unlike graphene on SiO 2 , using hBN, a new set of Dirac points is opened in the valence and conduction bands at the energy where the periodic potential connects the k and -k bands. [16e] The use of lowest-energy lattice configuration model shows a band gap of 50 meV is opened in graphene when on hBN. [19] Further, it has been shown that the presence of hBN can aid in heat spreading in field-effect transistors (FET) device geometries, thus reducing operating temperature. [20] In this paper, we report the differences in performance of solution-gated graphene ISFETs using thermal SiO 2 and polycrystalline chemical vapor deposition (CVD) hBN as the substrate for graphene, which here onwards are referred to as SiO 2 and hBN devices, respectively. Using solutions of varying concentrations of KCl and CaCl 2 , we characterize the changes in transfer curves for each of the devices in terms of Dirac voltage, transconductance, charge carrier concentration, and mobility. Results and DiscussionFigure 1a-c shows details of the device layout and setup for characterization. Further details have been included in the Experimental Section. Graphene and hBN were characterizedThe charge transport in solution-gated graphene devices is affected by the impurities and disorder of the underlying dielectric interface and its interaction with the solution. Advancement in field-effect ion sensing by fabricating a dielectric isomorph, hexagonal boron nitride between graphene and silicon dioxide of a solution-gated graphene field-effect transistor is being reported. Ionic sensitivity of Dirac voltage as high as −198 mV per decade for K + and −110 mV per decade for Ca 2+ is recorded. Increased transconductance due to increased charge carrier mobility is accompanied with larger ionic sensitivity of the transconductance due to larger ionic sensi...

This paper reports our findings on how to prepare a graphene oxide-based gas sensor for sensing fast pulses of volatile organic compounds with a better signal-to-noise ratio. We use rapid acetone pulses of varying concentrations to test the sensors. First, we compare the effect of graphene oxide deposition method (dielectrophoresis versus solvent evaporation) on the sensor’s response. We find that dielectrophoresis yields films with uniform coverage and better sensor response. Second, we examine the effect of chemical reduction. Contrary to prior reports, we find that graphene oxide reduction leads to a reduction in sensor response and current noise, thus keeping the signal-to-noise ratio the same. We found that if we sonicated the sensor in acetone, we created a sensor with a few flakes of reduced graphene oxide. Such sensors provided a higher signal-to-noise ratio that could be correlated to the vapor concentration of acetone with better repeatability. Modeling shows that the sensor’s response is due to one-site Langmuir adsorption or an overall single exponent process. Further, the desorption of acetone as deduced from the sensor recovery signal follows a single exponent process. Thus, we show a simple way to improve the signal-to-noise ratio in reduced graphene oxide sensors.

An intrinsic ion sensitivity exceeding the Nernst–Boltzmann limit and an sp 2 -hybridized carbon structure make graphene a promising channel material for realizing ion-sensitive field-effect transistors with a stable solid–liquid interface under biased conditions in buffered salt solutions. Here, we examine the performance of graphene field-effect transistors coated with ion-selective membranes as a tool to selectively detect changes in concentrations of Ca2+, K+, and Na+ in individual salt solutions as well as in buffered Locke’s solution. Both the shift in the Dirac point and transconductance could be measured as a function of ion concentration with repeatability exceeding 99.5% and reproducibility exceeding 98% over 60 days. However, an enhancement of selectivity, by about an order magnitude or more, was observed using transconductance as the indicator when compared to Dirac voltage, which is the only factor reported to date. Fabricating a hexagonal boron nitride multilayer between graphene and oxide further increased the ion sensitivity and selectivity of transconductance. These findings incite investigating ion sensitivity of transconductance in alternative architectures as well as urge the exploration of graphene transistor arrays for biomedical applications.

We have recently demonstrated enhancement in sensitivity of an impedance biosensor by seeding detonation nanodiamonds (DNDs) at the interdigitated electrodes (IDEs). Here, impedance spectroscopy of pure water is carried out at such IDEs to reveal the role of the DNDs. The impedance data is fit to an equivalent circuit model consisting of a geometrical resistance in series with a distributed element for Havriliak-Negami (HN) relaxation, and in parallel with a geometrical capacitance. Concurrently, the motion of charges across the IDEs is modeled as Plank-Nerst-Poisson anomalous diffusion across two partially blocking conductive electrodes. Results show that the DNDs (having a positive zeta potential) at the IDEs reduce the geometrical resistance

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